Generally, a brush less direct current (BLDC) motor has a longer lifespan, a more constant torque at a rated speed section, and significantly higher efficiency as compared with a direct current (DC) motor. In addition, since the BLDC motor includes a permanent magnet disposed in a rotor, dynamic characteristics are excellent, a brush and the rotor are not required, and a control range of a rotation speed is wide.
However, in order to drive the BLDC motor, a dedicated BLDC motor driver should be used.
The BLDC motor driver includes a speed controlling unit and a switching unit. The speed controlling unit generates a pulse width modulation (PWM) signal for driving the BLDC motor depending on a command rotation speed and a current rotation speed of the BLDC motor. The switching unit switches a direct current (DC) power depending on the PWM signal to generate a three-phase alternating current (AC) power and applies the generated three-phase AC power to the BLDC motor to drive the BLDC motor.
In the case in which the BLDC motor driver increases a rotation speed of the BLDC motor, the speed controlling unit may control a duty ratio of the PWM signal to rapidly increase the rotation speed of the BLDC motor to a required speed.
Further, in the case in which the BLDC motor driver decreases the rotation speed of the BLDC motor, the speed controlling unit may control a duty ratio of the PWM signal to decrease the rotation speed of the BLDC motor.
However, since rotation inertia of the rotor remains in the BLDC motor, a large time is required to decrease the rotation speed of the BLDC motor to a required speed in the case in which only the duty ratio of the PWM signal is controlled.
In addition, even though an input and an output of a current to and from the BLDC motor are completely suppressed by making the duty ratio of the PWM signal 0, it is difficult to rapidly react to a given speed decreasing command due to the rotation inertia of the rotor.
In the case of rapidly driving the BLDC motor at a decreased speed, a physical speed decreasing device may be used.
However, when the speed decreasing device is used, a physical configuration is very complicated, a separate cost is required, and an influence due to noise, vibrations, and the like, may not be avoided.